Silicon solar cells are devices for converting solar energy directly into electrical energy by means of photovoltaic effect. The aluminum paste used for silicon solar cells is a main member of the solar electronic pastes. In the manufacturing process of solar cells, the aluminum paste is usually printed on the backside of silicon wafers by means of screen printing, and aluminum is diffused into the silicon matrix to form a heavily doped P+ layer (i.e., a back surface electric field) after drying, sintering and cooling processes. Under the effect of the back surface electric field, electrons are pushed away from the back surface, and thus the recombination of electrons and holes is reduced, resulting in an increased open circuit voltage.
Furthermore, a mirror-like surface is formed at the backside of the solar cell after aluminum powder is sintered, which reflects sunlight unabsorbed by the solar cell back into the inside of the solar cell, resulting in an increase in the probability of photon absorption by the solar cell, and thus an increase in the short-circuit current of the solar cell. Therefore, the performance and properties of aluminum pastes are of vital importance for improving the photoelectric characteristics of solar cells.
Nowadays for the research and development of the backside aluminum paste, it is worth considering for skilled persons in the art to reduce the warpage of the silicon wafers in solar cells, besides to improve the photoelectric conversion efficiency of solar cells. Most surface of a silicon wafer is covered by the backside aluminum paste. During the sintering-cooling period, the shrinkage of the aluminum-silicon moiety is far greater than that of silicon moiety, since the linear thermal expansion coefficient of aluminum-silicon is far greater than that of silicon. Accordingly, the surface of the silicon wafer withstands a compressive stress, and thus, a warpage emerges. In addition, in order to reduce raw material costs, thickness decrease of silicon wafers is becoming more obvious. The warpage is more serious for the silicon wafers with thinner thickness.
In order to reduce the warping phenomenon, Chinese Patent Application No. 201010264080.3 disclosed a method wherein a coarse aluminum powder with a particle size of 6-8 μm and a fine aluminum powder with a particle size of 3-5 μm were used together, so as to reduce the stress generated in the sintering process, and thus to achieve a low warpage. However, the space generated by the expansion of the fine aluminum powder is limited, and thus the decrease of warpage is limited.